The temperature dependence of the spin-lattice relaxation of the radical pair of flavin and tyrosine in SyPixD was investigated by pulsed electron paramagnetic resonance (EPR) spectroscopy. Biphasic recovery with a fast component of the spin-lattice relaxation time, denoted as T-1fast, and slow component of spin-lattice relaxation time, denoted as T-1slow, was detected in the range of 4-80 K. Based on the temperature dependence of T-1fast and T-1slow below 16 K, the exchange coupling of the radical pair was estimated as 2J = 6-8 cm (1), defined as H = -2JS(1)*S-2. (C) 2010 Elsevier B. V. All rights reserved.